Disaggregation of a use: when splitting a unit needs consent

Splitting one planning unit into two or more separately occupied units can be development in its own right, even when the use class stays the same. That is disaggregation, and it catches more commercial occupiers than almost any other point in the Use Classes Order. A single shop let as two shops, an office floor carved into four tenancies with their own front doors, a house turned into two flats: in each case the council asks both what the building is used for and how many planning units now exist.

Section 55(1) of the Town and Country Planning Act 1990 defines development to include the making of any material change in the use of buildings or other land. Section 55(3)(a) then settles the residential case outright. The use as two or more separate dwellinghouses of a building previously used as a single dwellinghouse involves a material change of use, so subdividing a house into flats always needs planning permission. Everywhere else, materiality is a question of fact and degree for the local planning authority.

What disaggregation means in practice

The planning unit is the unit of occupation that planning law assesses. Courts have approached it by asking whether the occupier uses the whole site for a single main purpose, whether there are physically separate areas in distinct occupation, and whether any ancillary activity has grown into a use in its own right. Burdle v Secretary of State for the Environment set out that approach and officers still apply it.

Disaggregation happens when one unit of occupation becomes several. The total floorspace has not changed. The mix of activity might be identical on the day before and the day after. What has changed is the number of occupiers who can act independently, and that is what makes the planning consequences bite.

Think of a double-fronted retail unit of 180 m² let to one tenant, then relet as two units of 90 m². Nothing on the shelves is different. But the council now has two shopfronts, two sets of signage pressures, two waste streams, two delivery patterns and two future occupiers who may each want to change use again.

When subdivision becomes development

There is no floorspace threshold in the legislation. The council decides whether the change is material by looking at the planning consequences, and the honest answer in marginal cases is that it depends on the authority and the evidence put in front of it.

In my experience three factors move the decision:

• the degree of physical alteration, including new entrances, party walls, separate metering and separate sanitary provision
• whether the resulting units could operate wholly independently of each other
• whether the subdivision changes the demands placed on parking, servicing, refuse storage or residential amenity

Where the units remain interlinked, share a single entrance and a single service yard, and are held on one lease with internal partitions only, authorities often accept that the planning unit is unchanged. Where each part gets its own street door, its own rates assessment and its own tenant, the argument that one unit survives is weak.

Article 3(1) of the Town and Country Planning (Use Classes) Order 1987 says a change of use within the same class is not development. That is a protection for the use, not for the unit. A Class E(a) shop becoming two Class E(a) shops is still capable of being a material change of use, because the Order says nothing about the number of planning units.

Physical works and separate access

Separate access is the single clearest signal. A new door formed in a frontage is usually operational development needing permission in its own right, particularly on a commercial elevation or in a conservation area. Internal works to a building that is not listed are commonly outside planning control, so the subdivision argument has to rest on the use question rather than on the fabric.

For listed buildings the position is stricter. Works affecting character as a building of special architectural or historic interest need listed building consent whether or not they are internal, and partitioning a principal room to create two tenancies is exactly the kind of intervention a conservation officer will resist.

If your scheme needs supporting narrative on how the subdivision affects the building and the street, a Design and Access Statement is the document that carries it, alongside the planning case.

Effect on parking, servicing and standards

Once two units exist, most local plan standards apply to each of them. Parking standards expressed per unit rather than per square metre will generate a higher requirement after subdivision than before. Cycle parking, refuse and recycling storage, and loading provision are usually assessed the same way.

That final row is where councils get nervous. Two small units each below a local threshold may escape controls that the larger unit would have triggered, and a planning officer alert to that will say so in the report.

Where subdivision creates a servicing problem on a constrained street, the evidence a council expects is a parking and servicing note supported by survey work. A Parking Survey carried out to a recognised methodology is more persuasive than an assertion about on-street capacity.

Permitted development rights after subdivision

Rights can disappear the moment a unit is split. The General Permitted Development Order 2015 defines a dwellinghouse, for the purposes of Schedule 2 Part 1, so as to exclude a building containing one or more flats and a flat contained within such a building. Convert a house into two flats and the Part 1 householder rights for extensions, outbuildings, roof alterations and hard surfaces are gone from both.

Commercial rights follow a similar logic. Prior approval classes are written around a building or a use of a defined size, and the floorspace limits apply to the unit as it exists when the right is exercised. Splitting a building can take each part below a threshold, or it can take the arrangement outside the wording of the class entirely.

Check the condition and limitation wording of the relevant class against the post-subdivision position, not the position at purchase. This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it.

Evidence of separate occupation

Councils and, on appeal, inspectors look at hard documentary evidence rather than plans. The material that carries weight includes:

1. Leases or tenancy agreements showing the demised area and the term for each part
2. Business rates records showing separate assessments and the effective dates
3. Utility accounts and supply installation records
4. Dated photographs of separate entrances, signage and shopfronts
5. Trading records, invoices or bank statements tied to each occupier

An illustrative example. A unit in a parade is occupied as one retail shop until 2019, then let to a barber in the left half and a nail bar in the right, each with a separate rates assessment from 2020 and separate electricity supplies installed in that year. The owner wants certainty before refinancing. The rates list and the supply records are the backbone of the case, because they are third-party records with dates on them.

Regularising an existing split

Two formal routes exist. An application under section 191 of the 1990 Act for a lawful development certificate of existing use asks the council to certify what is already lawful, which matters where the split has run for long enough to be immune from enforcement. An application under section 192 asks the council to certify that a proposed use or operation would be lawful, which suits an owner who has not yet acted.

Alternatively apply for planning permission for the subdivision and accept conditions. That is often quicker and less costly where the evidence of past occupation is patchy, and it gives a clean decision notice for a buyer or lender to rely on. The trade-off is that you concede the change needed permission in the first place.

Either way, write the planning case around the planning unit question rather than around the use class. The Planning Statement should set out the unit as it was, the unit or units as they are, the physical and functional separation, and why the planning consequences are acceptable. PlanScope drafts that document from a structured set of answers about the site and the proposal, with sources cited and missing facts flagged rather than invented.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.